1. Field of the Invention
The present invention relates generally to liquid crystal display apparatus that uses an optically isotropic liquid crystal material, and more particularly to a transflective type of liquid crystal display apparatus.
2. Description of the Related Art
Display apparatus is a medium that visually conveys information to humans, and in the modern age with an advanced information society, the display apparatus is an entity important to humans and society. In recent years, liquid crystal display apparatus, in particular, has significantly improved in performance, and is adopted as display devices for a variety of products from hand-held telephones to personal computers and even to large-screen televisions and the like. Liquid crystal displays generally comprise a liquid crystal display panel and a backlight (illuminating device) disposed on the rear face of the liquid crystal display panel in order to irradiate this panel with light. For color image display, one pixel comprises three subpixels associated with, for example, the three primary colors (red, blue, and green), and the subpixels associated with each color are each independently controlled to reproduce various colors.
Because of its applicability to a wide range of illuminating environments, transflective-type liquid crystal display apparatus is used as liquid crystal displays in hand-held telephones or other mobile communication apparatus. The transflective type of liquid crystal display apparatus has a transmission area and a reflection area in one subpixel forming the display area of the liquid crystal display panel. The transmission area is provided to implement transmissive display by controlling the amount of transmission of the light emitted from the backlight. The reflection area is provided to achieve reflective display by controlling the amount of ambient light transmitted. That is to say, the transflective type of liquid crystal display apparatus can be used in a wide variety of illuminating environments since the display apparatus ensures the visibility of display images by achieving transmissive display primarily in dark environments, and reflective display primarily in bright environments. Liquid crystal displays of the IPS (In-Plane Switching) scheme are traditionally known to provide wide viewing angles. Liquid crystal displays of other schemes such as the VA (Vertical Alignment) scheme or TN (Twisted Nematic) scheme are also known. Irrespective of the kind of scheme adopted, each liquid crystal display comprising one pair of transparent substrates, a liquid crystal layer held in sandwiched form between the pair of transparent substrates and formed of a nematic liquid crystal material, and one pair of polarizers disposed on the respective faces of each transparent substrate that are formed at positions opposite to the liquid crystal layer, controls the amount of transmission of light by changing the polarization state of the light entering the liquid crystal layer, and thus displays an image.
As described in Non-Patent Document 1 listed below, nematic liquid crystal materials exhibit a light-scattering behavior due to thermal fluctuation of molecules. In both the IPS scheme and the VA scheme, display panels that employ a nematic liquid crystal material assume a black state when no voltage is applied. During operation, the corresponding display apparatus, whether it be of the IPS or VA scheme, is reduced in contrast ratio by leakage of light due to the scattering thereof. The reduction in contrast ratio is among the characteristic problems of the nematic liquid crystal materials.
In contrast to this, liquid crystal materials with optically three-dimensional or two-dimensionally optical isotropy are known in recent years (hereinafter, these materials are referred to simply as isotropic liquid crystal materials). These isotropic liquid crystal materials have the properties that when no voltage is applied to the liquid crystal layer, the liquid crystal molecules aligned will have optically three-dimensional or two-dimensionally optical isotropy, and that when an electric field is formed by the application of a voltage, birefringence will be induced in the direction of the electric field. The kinds of isotropic liquid crystal materials reported in recent years include a three-dimensionally isotropic type exhibiting a smectic blue phase or a cholesteric blue phase. These kinds of materials further include a two-dimensionally isotropic type having a bent-core structure. The type with the bent-core structure has a liquid crystal compound aligned vertically with respect to the substrates, and exhibits isotropy in the plane of the liquid crystal layer under no voltage. Other known materials include those which exhibit either a cubic phase, a smectic Q-phase, a micelle phase, a reverse micelle phase, a sponge phase, or the like.
Materials of the blue phases have traditionally been extremely narrow in applicable temperature range, and hence, low in practical applicability. Non-Patent Documents 2 and 3 listed below describe the extension of the applicable temperature ranges in the materials of the blue phases. Also, Non-Patent Document 4 listed below describes isotropic liquid crystal materials and the properties thereof, such as optical biaxiality in the bent-core structure. Non-Patent Document 5 below describes the optical elements that use an isotropic liquid crystal.
Additionally, JP-A-2006-3840 discloses a detailed electrode structure of a liquid crystal display panel using an isotropic liquid crystal. In JP-A-2006-215287 disclosing a transflective liquid crystal display apparatus that uses an isotropic liquid crystal, it is described that the apparatus is configured so that an electric field in a reflection area is weaker than that of a transmission area. More specifically, it is described that the distance between a plurality of electrodes constructed to form an electric field essentially parallel to a substrate surface is changed to differ between the reflection area and the transmission area.    Non-Patent Document 1: Physical Properties of Liquid crystals, pp. 90-94, written by W. H. de Jeu, translated by Chikara Ishii and Shunsuke Kobayashi    Non-Patent Document 2: Harry J. Coles, NATURE, Vol. 436, pp. 997-1000, 2005    Non-Patent Document 3: Atsushi Yoshizawa et al., Journal of Materials Chemistry, Vo. 15, pp. 3285-3290, 2005    Non-Patent Document 4: Bharat R. Archarya et al., LIQUID CRYSTALS TODAY, VOL. 13, No. 1, pp. 1-4, 2004    Non-Patent Document 5: Hirotsugu Kikuchi, Advanced Materials, Vol. 17, pp. 96-98, 2005